Conversion of a beard trimmer from nimh to Li-ion

[downloaders]

I'm an electronics enthusiast (so I don't REALLY know what I'm doing) but think my idea will work, but as usual when dealing with an idea for the first time, there is always room for improvement 

The motivation for this is my new trimmer is not half as good as this one was, so I decided to resurrect it.

The trimmer I have is a philips QT4022, and somehow when its fully charged (I swapped the original aaa with eneloops and charged them inside AND outside the unit) it blinks amber (sign for empty battery)



In a nutshell, this is my plan:


Notes:
- The button is a momentary push switch and I would like to keep it, that's the reason for the push switch circuit
- 2 aaa's would be around 3v I guess, that's why I went for the 3v output (correct me if I'm wrong)

Step down: https://www.ebay.com/itm/272666856859
Switch Circuit: https://www.ebay.com/itm/172993257617

Any suggestions for other circuits are welcome, as I just picked them because they suited the needs and they are maybe not the best for the job.

Is there something wrong with my plan, or any way to improve it or make it simpler?

Thanks

[james_s]

I would suggest looking for a lithium iron phosphate cell instead, they are around 3.6V instead of 4.2V and should work fine powering the motor directly and if you get an internally protected cell you can dispense with the separate protection against over-discharge. Small DC motors like that will work well over quite a wide voltage range usually.

[Audioguru]

where is the Li-ion battery charger circuit?

[downloaders]

@ james_s: A lifepo4 would be better, but I have none, have a ton of 18650 around and where i live it would be expensive to get one. Or do you mean the circuit doesn't work this way and that is the only option?

@ Audioguru: The charging would look something like this:



I suppose it should work, but i'm not sure.

[Audioguru]

Your charger looks like it is designed for AA Ni-MH cells, not Lithium-Ion cells. The charger circuits are completely different.
Correction; I looked up your Banggood Chinese battery charger and it might be very good if it does not fail soon.

[6PTsocket]

I would suggest looking for a lithium iron phosphate cell instead, they are around 3.6V instead of 4.2V and should work fine powering the motor directly and if you get an internally protected cell you can dispense with the separate protection against over-discharge. Small DC motors like that will work well over quite a wide voltage range usually.

You are talking about peak voltage, fresh off the charger. An 18650 and it's chemically similar cousins are nominally 3.6 volts over almost the whole discharge curve. The phosphate cells are proportionally lower as well. Even the original NiMH and NiCd cells are higher than 1.2 volts,  fresh off the charger. All rechargables are higher than nominal after a full charge.

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[amyk]

I would suggest looking for a lithium iron phosphate cell instead, they are around 3.6V instead of 4.2V and should work fine powering the motor directly and if you get an internally protected cell you can dispense with the separate protection against over-discharge. Small DC motors like that will work well over quite a wide voltage range usually.

They'll work very well, and can even be "overvolted" a bit for more performance, but lifetime is reduced considerably: https://www.pololu.com/docs/0J11/all
The main wear item is the brushes, which on these small motors can last less than a day of continuous operation even at rated voltage.

[BradC]

They'll work very well, and can even be "overvolted" a bit for more performance, but lifetime is reduced considerably: https://www.pololu.com/docs/0J11/all
The main wear item is the brushes, which on these small motors can last less than a day of continuous operation even at rated voltage.
[/quote]

The motors in that test are the cheapest of the cheap with bent metal brushes. They'll last a lot longer with some suppression caps on them to reduce the brush arcing. There are always more expensive variants available with carbon block brushes that last considerably (like several orders of magnitude) longer, but you get what you pay for. They also handle more voltage a bit more gracefully.

I'd be super surprised if the motor in that beard trimmer didn't have carbon brushes and some form of suppression.

[Zero999]

The motors in that test are the cheapest of the cheap with bent metal brushes. They'll last a lot longer with some suppression caps on them to reduce the brush arcing. There are always more expensive variants available with carbon block brushes that last considerably (like several orders of magnitude) longer, but you get what you pay for. They also handle more voltage a bit more gracefully.

I'd be super surprised if the motor in that beard trimmer didn't have carbon brushes and some form of suppression.

Metal brushes aren't used to save money, but are better suited to lower voltages. Carbon brushes on a 3V motor would result in inferior efficiency to metal brushes.

[james_s]

You are talking about peak voltage, fresh off the charger. An 18650 and it's chemically similar cousins are nominally 3.6 volts over almost the whole discharge curve. The phosphate cells are proportionally lower as well. Even the original NiMH and NiCd cells are higher than 1.2 volts,  fresh off the charger. All rechargables are higher than nominal after a full charge.

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Yes I know that, and we are talking about a 3V motor here which was probably run from a 2 cell NiMH pack. If it was indeed powered by a 3 cell pack then the higher voltage LiCo cell would be a good option. 18650 is a physical form factor, not a cell type. While most 18650 cells are LiCo there are exceptions and LiFePo4 is available. The thing I like about the phosphate type is that they don't mind being stored fully charged for extended periods and they tend to be quite good at supplying high current.

[PointyOintment]

The motors in that test are the cheapest of the cheap with bent metal brushes. They'll last a lot longer with some suppression caps on them to reduce the brush arcing. There are always more expensive variants available with carbon block brushes that last considerably (like several orders of magnitude) longer, but you get what you pay for. They also handle more voltage a bit more gracefully.

I'd be super surprised if the motor in that beard trimmer didn't have carbon brushes and some form of suppression.

Metal brushes aren't used to save money, but are better suited to lower voltages. Carbon brushes on a 3V motor would result in inferior efficiency to metal brushes.

Is there a way to tell what brush material a motor has without opening it (assuming you can't peek inside through a vent hole, and don't have an X-ray machine)?

[james_s]

Is there a way to tell what brush material a motor has without opening it (assuming you can't peek inside through a vent hole, and don't have an X-ray machine)?

If there's a part number on it you might be able to find a datasheet.

[downloaders]

I don't actually understand everything you guys are saying but regarding the "An 18650 and it's chemically similar cousins are nominally 3.6 volts over almost the whole discharge curve" I don't understand where the data for this comes from. I use 18650 and Lipo's a lot and with this little motor I guess the 18650 will be at least at 3.8V for quite some time. The AAA were 900mah and lasted maybe an hour, a 3000mah 18650 will last maybe 2.5hours and I guess for close to being empty it will output >3v.

[james_s]

I don't actually understand everything you guys are saying but regarding the "An 18650 and it's chemically similar cousins are nominally 3.6 volts over almost the whole discharge curve" I don't understand where the data for this comes from.

https://batteryuniversity.com/learn/article/lithium_based_batteries

Again, "18650" is not a battery chemistry, it is a physical for factor, 18mm diameter, 65mm long. Multiple battery types are made in the 18650 size.

[downloaders]

So the step down module came, and guess what? I does not work. I should have measured the current. The module goes up to 500ma and When measured, the motor needs max 1.5A when i hold it down a bit like when cutting beard.

I couldn't find any step down module that could turn a 4.2 - 3.5V to 3V with 1.5A or more.

Does anybody know one that I could use, or some other way to bring the voltage down to 3v from 4.2 - 3.5V?

Thanks!!

[james_s]

Should be lots of different modules that will do that.

I would try just running the motor direct from the battery though, most small DC motors work nicely over quite a large voltage range. Otherwise look for a LiFePO4 cell, lower voltage than most Li-ion types.

[6PTsocket]

A silicon diode has a typical forward drop of .7 volts. Two in series drops 1.4 volts.Not very elegant and wastes a little power. Just make sure you use diodes that can handle the current. The 1N4xxx series are only good for an amp. You'll have to look it up.

So the step down module came, and guess what? I does not work. I should have measured the current. The module goes up to 500ma and When measured, the motor needs max 1.5A when i hold it down a bit like when cutting beard.

I couldn't find any step down module that could turn a 4.2 - 3.5V to 3V with 1.5A or more.

Does anybody know one that I could use, or some other way to bring the voltage down to 3v from 4.2 - 3.5V?

Thanks!!

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[6PTsocket]

I apologize if I am hijacking the thread but I have a very similar situation. I have an old Iso tip soldering pencil. The power is a 2 cell NiCd sub C pack rated for maybe 1300mah. It has a 1 hr charger. There is a thermal switch mounted against the battery that trips when the temp rises at full charge. This was a common charge method with NiCds and cheaper than more sophisticated methods. The switch is bypassed with a resistor to maintain a trickle charge. Trickle charging is the death of NiCd's and the real reason for loss if capacity, not memory. The memory problem was corrected in the early days of NiCd production but the story stuck, according to Battery University.The battery is long dead. I can put in some 2Ah  sub C's and charge with my smart charger that does no need any thermal sensing but as I use the iron so seldom, I thought I might want to use an 18650. The lithiums in my Bosch tools seem to stay charged forever. My concern is what to do with the extra 1.2 volts. I could use a couple of diodes and get close but that is wasteful and maybe too much heat inside the case. Unlike a motor, the soldering tips wjth built in element might not tolerate the extra voltage. If there is no good solution I will stick with better NiCds and my smart charger.

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